在 (zài) and 正在 (zhèngzài) can be used as auxiliary verbs to express that an action is ongoing or in progress. This is often the equivalent of present continuous in English, which is how we express that an activity is happening now.

You can use 正在 (zhèngzài) instead of just 在 (zài) to put a little more emphasis on an action that is in progress right now.

There is no need to worry too much about when to use 在 (zài) or 正在 (zhèngzài), since they basically mean the same thing. 正在 (zhèngzài) usually shows that the action is in progress (right now). "在 (zài) + Verb" is more commonly used than "正在 (zhèngzài) + Verb," but both are fine to use.